A novel approach to characterizing localized corrosion within a crevice

Abstract

A method of experimentally simulating crevice corrosion was developed that allows the nominal current density to be determined in a straightforward manner, in contrast to a gap crevice. The apparatus involves placing a crevice cover with a hole oriented perpendicular to the specimen and then measuring the current between the specimen and a counter electrode with a zero resistance ammeter and the potential versus a reference electrode. The effects of diffusion and polarization can be largely separated by the presence or absence of a crevice cover and the resistance to transport in the crevice can be controlled by the depth of the crevice cover. Test specimens were made of nickel aluminum bronze (NAB) materials that are used in marine applications. For this material the corrosion rate was influenced by the cathodic rate in a straightforward manner and by diffusion effects caused by the crevice. The presence of a crevice produced a positive shift in the specimen potential that is consistent with an increase in the potential of the controlling anodic reaction due to higher concentrations of dissolution products at the metal surface. Surface roughness observations after exposure suggest a correlation with the skewness of the current density. The skewness measurements of the de-trended current density of the five NAB samples examined suggest the following: (i) a crevice increases the propensity of a NAB towards localized corrosion, (ii) a longer crevice length increases the propensity of NAB towards localized corrosion and (iii) laser treatment can greatly reduce the propensity of NAB towards localized corrosion in a crevice.